The monitoring of therapeutic drug levels in biological fluids such as serum, plasma, whole blood, urine, and the like, has become very useful to provide physicians with information to aid in patient management. The monitoring of such drug levels enables adjustment of patient dosage to achieve optimal therapeutic effects, and helps avoid either subtherapeutic or toxic levels. Doxepin is a tricyclic antidepressant drug which exists in two isomeric forms, E-doxepin (Formula IA, R.dbd.CH.sub.3) and Z-doxepin (Formula IB, R.dbd.CH.sub.3): ##STR2##
Although doxepin has been found to be very effective in treating chronic depression, its concentration in a patient's blood must be maintained in a therapeutic range. A wide interpatient variation normally exists in human plasma for a given dose. However, high doses have been associated with central nervous system disorders, toxicity, hypertension, seizures, coma and death. Since individuals vary greatly in their response to doxepin, it is necessary to monitor the therapy by measuring the level thereof in, for example, the serum or plasma of the patient.
Doxepin is administered as a mixture of the E-doxepin and Z-doxepin isomers, also referred to as trans-doxepin and cis-doxepin, respectively, at a ratio of about 85:15 (E:Z). Once administered, doxepin is metabolized by N-demethylation to form desmethyldoxepin, which is also active and which also occurs as both the E and Z isomers thereof (Formulae IA and IB, R.dbd.H). For des-methyldoxepin, varying ratios of the isomers have been reported in individual patients.
Since both doxepin (E-isomer and Z-isomer) and desmethyl-doxepin (E-isomer and Z-isomer) are active for treating depressive symptoms, conventional diagnostic techniques for the determination of doxepin levels is based on the measurement of the levels of the respective isomers of doxepin and desmethyldoxepin wherein the therapeutic range ascribed to blood levels in a patient is the total of doxepins and desmethyldoxepins, i.e. E-doxepin plus Z-doxepin plus E-desmethyldoxepin plus Z-desmethyldoxepin equals total doxepins. Concentrations below the range are proposed to be subtherapeutic for the treatment of depression, while levels higher than the range can be associated with undesirable effects including cardiovascular complications, anticholinergic effects, and sedation, without any increase in antidepressant efficacy.
Although the levels of doxepins and desmethyldoxepins can be measured by chromatographic techniques, such as high pressure liquid chromatography [Park, J. of Chromatogr., 375, 202-206 (1986)] or gas chromatography [Rosseel et al., J. Pharm. Sci., 67, 802-805 (1978)], such techniques are labor intensive, requiring highly skilled personnel to perform various cumbersome steps which are time consuming and tedious. Similarly, derivatives of the tricyclic antidepressant drug known as amitriptyline have been employed for the generation of antisera and labeled reagents for use in a radioimmunoassay system for determining doxepin and desmethyldoxepin levels [Midha and Charette, Communications In Psychopharmacol., 4, 11-15 (1980); Virtanen, et al., Acta. Pharmacol. Et. Toxical., 47, 274-278 (1980)]. However, such techniques employing non-isomeric immunogens (i.e., linking an N-substituted amitriptyline to a protein carrier) and non-isomeric labeled reagents have failed to demonstrate equivalent recognition of the isomers of doxepin and desmethyldoxepin. In particular, a non-specific fluorescence polarization immunoassay (FPIA) for the detection of the total amount of the four major tricyclic antidepressant drugs is commercially available and described in European Patent Application Publication No. 226,730 and U.S. Pat. No. 4,420,568 wherein the concentration determined by this assay is only a crude estimation of the total amount of tricyclic antidepressant in plasma or serum. Accordingly, such assay cannot be used to accurately quantify the total amount of all four isomers of doxepin and desmethyldoxepin.